It is common practice to control particulate emissions from boilers and similar devices by directing particle-laden flue gases through separating devices for removing a large percentage of the particles prior to directing the gases to atmosphere. For this purpose cyclone separators have been employed; however, in many localities the degree of particle removal employing only cyclone separators has not been found satisfactory to meet stringent air pollution requirements.
Scott Paper Company, the assignee of the instant application, employs several boilers that burn wood waste as fuel (i.e. hogged fuel boilers) to provide heat and steam for its manufacturing operations in Everett, Wash. This use of wood waste is desirable for at least two reasons. First, it is a relatively inexpensive source of energy, as compared to petroleum fuel products, and second, it eliminates a potential disposal problem for the wood waste materials. However, it has been difficult to consistently meet stringent local air pollution requirements by merely directing the spent flue gases from the hogged fuel boilers through cyclone separators prior to emitting the gases to atmosphere. To meet these stringent air pollution requirements Scott Paper Company has installed an extremely efficient baghouse system downstream of its existing cyclone separators for removing the necessary particulate contaminates from the gaseous stream prior to directing the gas to atmosphere.
Baghouses are well known devices employed to separate particulate materials from gases. One conventional system (i.e. the one employed by Scott Paper Co.) is manufactured by Standard Havens of Kansas City, Mo., and includes a plurality of filter bags formed of woven fiberglass, coated with Teflon. The baghouse system utilized by Scott in conjunction with five (5) hogged fuel boilers includes seventeen modules; each housing 210 bags. The flue gases from the boilers, after being directed through cyclone separators, are directed into the baghouse and through the filter bags prior to being emitted through the stack to atmosphere. Fine particulate materials that are not removed in the cyclones are trapped on the outer surfaces of the filter bags. At periodic intervals different modules can be taken off-line and either shaken or pulsed with air to free the particulate material from the filter bags and cause the freed material to fall by gravity out the bottom of the modules.
Several different systems have been employed to convey fly ash and similar particulate materials away from baghouse modules, cyclone separators and similar particle separating devices.
For example, screw conveyors and chain conveyors have been employed in connection with rotary valves at the lower end of hoppers associated with baghouse modules to convey particulate material away from the baghouse. However, when the baghouse is operated in a corrosive, high temperature environment, such as is encountered at a paper mill, bearings associated with the screw and chain conveyors tend to wear excessively; requiring undesirable downtime and expensive bearing replacements. In addition, the rotary valves which control the feed of particles to the conveyor tend to plug up and malfunction; thereby causing the discharge opening from the modules to plug up. This can cause the modules to fill up with hot fly ash and other particulate material which burn the filter bags thereby creating a costly bag replacement problem. Furthermore, the rotary valves which tend to wear out or seize-up in corrosive high temperature environments of the type encountered at a paper mill, are expensive to replace.
A different type of conveying system utilizes compressed air as the conveying medium. In this system a rotary valve is utilized to feed the particulate material into the stream of compressed air, and to isolate the compressed air from the particle-separating device so that it will not interfere with the particle-separating operation. The use of rotary valves in this system introduces the same problems described above in connection with the chain and screw conveying systems. In addition, the compressed air added to the system to accomplish the removal operation requires the use of more complex air handling and separating equipment than would be the case if air were not added to the system.
Slurry systems also have been employed to convey particulate materials; particularly in connection with cyclone separators operated under positive pressure. The gases containing particulate materials are received in the separators under positive pressure, and the heavier particles are centrifuged to the outer walls and dropped to the bottom by gravity. These heavier particles are dumped into an exit conduit filled with flowing water to convey the particles to further storage and/or separation systems. Although this type of slurry system had been found to work well in the conveying of heavy particles separated in cyclone separators operated under positive pressure; it is extremely difficult to employ such a slurry system in a high temperature environment for conveying away lighter particulate material from a baghouse or similar installation operated under negative-or atmospheric pressure conditions. In these latter systems the water in the conveying duct tends to vaporize, and the vapors tend to enter the modules associated with the baghouse to wet and clog the filters; thereby impairing the particle separating operation of the unit. This problem is particularly acute when the modules are operated under negative pressure conditions.
If a slurry conveying system could be designed for use with baghouses and other particle-separating or storage devices maintained either under atmospheric or negative pressure conditions without the attended vaporization problems, and without the need for incorporating rotary or similar valve structures into the devices, the slurry system would clearly be more preferred than their mechanical counterparts. Specifically, such a slurry system would not be riddled with the problems of worn bearings and corroded parts that are encountered in the prior art mechanical systems employing screw or chain conveyors and rotary valve systems. It is to this latter type of slurry system that the instant invention is directed.